Source:

Response:

Darwin wrote a letter declining the dedication of an unnamed book on
atheism, but he wrote it to Edward Aveling. Aveling's common-law wife
was Elanor Marx, Karl's daughter, and she inherited his papers. They
got mixed up with Karl Marx's papers, and the letter was assumed to
have been to Marx. This view found ideological favor in Russia, so it
was widely repeated. Later, a letter from Aveling, requesting
permission to dedicate his book The Student's Darwin to Darwin, was
found among Darwin's papers.
Darwin declined permission and argued that science should not address
religious matters directly (Colp 1982; Carter 2000).

Darwin did have a copy of Das Kapital, but its pages were unseparated
when he died, so he never read it.